1. Field of the Invention
The present invention relates to a video data playback system capable of operating in a normal playback mode and a fast (forward and reverse) playback (visual search) mode, and more particularly, to a scheme for producing coded video data for the fast playback mode in this type of video data playback system, which can be utilized for video data input at a center (video server) of a video-on-demand (VOD) system to realize interactive video data playback between a center and a terminal via a communication channel, or for video data input at a stand-alone type video disk drive device.
2. Description of the Background Art
The video-on-demand service for providing video data is realized by connecting a video server for storing coded video data and a set-top device for reproducing video data from the coded video data via a communication path.
For this video-on-demand service, there is a proposition as disclosed in U.S. Pat. No. 5,479,303 for a scheme to realize a fast playback (visual search) mode similar to that provided in a usual VTR device, in which normal playback coded video data and fast playback coded video data are separately stored in advance in the video server, and coded video data to be used for video data reproduction are switched among these two types of coded video data whenever a normal playback request or a fast playback request is issued by the set-top device.
In a case of realizing this video-on-demand service with the fast playback mode, the normal playback coded video data are to be encoded by a video coding scheme using both intra-frame coding and inter-frame coding in view of the coding efficiency, whereas the fast playback coded video data are to be encoded by a video coding scheme using only the intra-frame coding that enables random access at any desired frame, because when the set-top device issues the fast playback request is not known in advance.
Now, in a video data playback system capable of operating in a normal playback mode and a fast (forward and reverse) playback (visual search) mode, the coded video data required in realizing the fast playback mode are to be produced by a coded video data production device and stored in a storage device such as a video server in advance.
In this regard, the conventionally known schemes for producing the fast playback coded video data are as follows.
A first conventional scheme is a scheme in which the same video source is reproduced at a normal speed by a normal playback device as well as at a fast speed by a fast playback device, and each video frame reproduced at a normal speed is encoded by an encoder for a normal playback mode so as to produce the normal playback coded video data, while each video frame reproduced at a fast speed is encoded by an encoder for a fast playback mode so as to produce the fast playback coded video data.
In other words, independent playback devices and independent encoders are provided for the normal playback and fast playback modes, and the normal playback coded video data are produced by the playback device and the encoder for a normal playback mode while the fast playback coded video data are produced by the playback device and the encoder for a fast playback mode, both from the same video source independently.
In this first conventional scheme, the normal playback coded video data and the fast playback coded video data are to be produced by independent processing systems, so that in principle it may become impossible to link these normal playback and fast playback coded video data properly when there is even a slight difference or variation in clocks of these two processing systems.
For example, if there is 1% error in clocks of these two processing systems, when the normal playback and fast playback coded video data for a video source with 100 minutes of reproduction time are produced by accurately aligning them at the beginning, there is going to be an error of one minute in links between them at the end of the video source. If there is a request for switch-over from the normal playback to the fast playback at this point, the playback point would be displaced by one minute when the playback mode is switched in response to the request.
In addition, in this first conventional scheme, there is a structural difficulty related to the fast playback device. Namely, the tape recording format used by a usual analog VTR presupposes a use of a normal playback, so that stripe like noises are often produced at a time of the fast playback, and in order to realize the video data playback without such noises even at a time of the fast playback, it is necessary for the playback device to have a special structural configuration for a noise reduction which is not provided in a usual playback device.
On the other hand, a second conventional scheme is a scheme which uses a configuration as shown in FIG. 1, in which each video frame is encoded by an encoder 42 to produce the normal playback coded video data 43 first, and then the normal playback coded video data 43 so obtained are entered into a filter 44 to extract only selected frames of the normal playback coded video data to produce the fast playback coded video data 45.
In this second conventional scheme, several problems arise when the encoder 42 employs a video coding scheme such as MPEG (Moving Picture Experts Group: ISO/IEC11172) which uses both the intra-frame coding and the inter-frame coding.
FIG. 2 shows an exemplary manner of producing the fast playback coded video data 45a in a case of using MPEG as the video coding scheme at the encoder 42.
Namely, each original video frame 41a is encoded by the encoder 42 to produce the normal playback coded video data 43a first. Here, a symbol such as I1, B2, P4, etc. shown in each frame of the normal playback coded video data 43a indicates a frame type and a number of frames from the top for each frame. For instance, I1 indicates that it is the coded data for I-frame which is the first frame from the top, B2 indicates that it is the coded data for B-frame which is the second frame from the top, P4 indicates that it is the coded data for P-frame which is the fourth frame from the top, and so on.
Note that the normal playback coded video data 43a encoded by MPEG include a coded frame data called I-frame which only contains data in an original frame and which is produced by the intra-frame coding, and coded frame data called B-frame and P-frame which contain data on difference between the original frame and the I-frame and which are produced by the inter-frame coding. Here, the I-frame can be decoded into the original frame from the coded frame data of this I-frame itself alone, but each of the P-frame and the B-frame cannot be decoded into the original frame without using the coded frame data of the I-frame.
Now, consider a case of producing the fast playback coded video data 45a from the normal playback coded video data 43a encoded by MPEG as described above. In the fast playback control using the fast playback coded video data 45a, when there is a fast playback request during the normal playback, it is necessary to realize the fast playback by switching the coded video data to be reproduced from the normal playback coded video data 43a at that point to a corresponding point of the fast playback coded video data 45a, so that it is necessary for the fast playback coded video data 45a to be capable of starting the fast playback from an arbitrary point.
For this reason, it is necessary to form the fast playback coded video data 45a by I-frames in which every frame is encoded by the intra-frame coding. In other words, the fast playback coded video data 45a are produced by extracting only I-frames by means of the filter 44.
Here, however, the I-frame is produced by using only the data in the original frame at a time of the encoding, so that its data size is considerably larger than that of the P-frame or the B-frame in general, and consequently the fast playback coded video data 45a so produced by extracting only the I-frames has a considerably larger data size per frame compared with the normal playback coded video data 43a.
For instance, when the data sizes for the same six frames are compared in FIG. 2, it can be seen that the fast playback coded video data 45a has about 2.5 times greater data size than the normal playback coded video data 43a. This implies that, if the normal playback and the fast playback are to be made by the same frame rate (i.e., a case of full motion fast playback) using the normal playback coded video data 43a and the fast playback coded video data 45a, the playback bit rate in a case of the fast playback would be 2.5 times as much as the playback bit rate in a case of the normal playback.
Such a difference in the playback bit rate depending on the playback mode can be very inconvenient in a case of constructing the video data playback device.
Namely, in a case of using a stand-alone type video data playback device such as a video disk drive device, for example the transfer bit rate from the video disk drive device is going to be different for the normal playback and the fast playback, so that it becomes necessary to provide an additional mechanism to control the rotational speed of the disk according to the playback mode.
Also, in a case of using the video-on-demand service system for providing the coded video data via a communication channel, it becomes necessary to provide a complicated communication channel resource management capable of dynamically changing the communication channel rate according to the playback mode.
Moreover, in order to cope with a case in which many terminals utilizing the video-on-demand service issue fast playback requests simultaneously, it is also necessary for a communication channel provider to provide extra channel resources in excess to the usual need.
Furthermore, in this second conventional scheme, the filter 44 for producing the fast playback coded video data 45a is required to extract the I-frames only, so that the fast playback speed to be realized by the fast playback coded video data 45a can be set only to an integer multiple of the I-frame interval in the normal playback coded video data 43a.
For example, in a case of FIG. 2, the I-frame appears in every six frames in the normal playback coded video data 43a, so that the six times faster playback is realized by extracting the I-frame from every six frames in a sequence of I1, I7, I13, and so on. In this case, the other possible settings of the fast playback speed are limited only to integer multiples of six, such as the twelve times faster playback that can be realized by extracting the I-frame from every twelve frames in a sequence of I1, I13, I25, and so on.